1. Field of the Disclosure
Embodiments disclosed herein relate generally to downhole tools. In particular, embodiments disclosed herein relate to methods and assemblies for drilling a deviated secondary borehole from an existing borehole in geologic formations.
2. Background Art
Traditionally, whipstocks have been used to drill a deviated borehole from an existing earth borehole. The whipstock has a ramp surface which is set in a predetermined position to guide the drill bit on the drill string in a deviated manner to drill into the side of the earth borehole until a secondary borehole is established branching from the existing borehole. In operation, the whipstock is located at a desired depth in the existing borehole by one of several techniques (combined with an anchor or packer that can be set to a desired location in a borehole, set on bottom, or set on plug, etc.), the whipstock is then surveyed when at the desired depth so that the whipstock face can be oriented azimuthally. Then the drill string is lowered into the well and the whipstock serves to deflect or urge the drill bit into the side wall of the bore hole at the angle of the ramp on the whipstock. As the drill string progresses, a secondary borehole is drilled that deviates or branches from the existing borehole. This process of drilling a deviated secondary borehole from an existing borehole is also called “sidetracking.” And more specifically, it may be referred to as open-hole sidetracking when the existing borehole is uncased, that is, the side-tracking initiates directly into the earth formation side wall of the existing borehole.
The typical open-hole sidetracking operation requires at least two trips. The first trip generally involves orienting and setting the whipstock in the open borehole. The second trip generally involves going back into the hole with the bottom-hole-assembly (BHA) with a conventional drill bit to drill the deviated secondary borehole. It is also common that an open hole-side tracking operation requires more than two trips. The anchor or packer may be set in one trip without the presence of the whipstock which is then oriented and set in a separate trip. Or there may be issues with initiating the deviated borehole which requires multiple trips to get started. In general, the elimination of “trips” to accomplish an operation is desirable as each “trip” of the drill string or work string in and out of a well is timely which results in delay and higher drilling costs.
It is also common to drill deviated boreholes from a cased borehole—known as cased-hole sidetracking. The following patents, commonly owned by the current assignee, disclose systems for one-trip milling of cased-hole deviated boreholes: U.S. Pat. Nos. 5,771,972; 5,894,889; 6,102,123; 6,648,068; and 7,207,401. This family of patents also discloses a method that allows “drilling ahead” into the formation further than the typical rat-hole and preferably to the desired target depth (“TD”) of the customer. The configuration and design of the mills that are used to “drill ahead” are a balance of being able to handle the challenging demands of milling the window in the casing and being able to drill to TD with a reasonable rate of penetration (“ROP”). As such, the mills used in these cased-hole sidetracking systems have not been conventional drill bits that, in conventional drilling, are selected or designed to focus on the efficient drilling of the particular lithologies of the earth formation expected to be encountered while drilling to TD.
Accordingly, there exists a need for a one-trip sidetracking tool for sidetracking operations in uncased boreholes.